Many high speed data transmission networks rely on transceivers, including optical transceivers and similar devices, for facilitating transmission and reception of digital data embodied in the form of, for example, optical signals over optical fibers. Typically, data transmission in such networks utilizes an electro-optic transducer emitting light when current is passed there through with the intensity of the emitted light being a function of the current magnitude through the transducer. Data reception is generally implemented by way of an optical receiver (also referred to as an optoelectronic transducer), an example of which is a photodiode. The optoelectronic transducer receives light and generates a current, the magnitude of the generated current being a function of the intensity of the received light.
During the operation of an optical transceiver, it is often important to evaluate the quality of a received data signal. One tool often used to help in the evaluation process is an eye diagram or pattern. As is well known, an eye diagram is formed by superimposing a long stream of random bits on one another on an oscilloscope or like device. The bit streams include the transitions from high to low and low to high. Several system performance measures can be derived by analyzing the eye diagram. For example, if the signals are too long, too short, poorly synchronized with the system clock, too high, too low, too noisy, too slow to change, or have too much undershoot or overshoot, this can be observed from the eye diagram. In particular, an “open” eye diagram corresponds to minimal signal distortion.
As stated, the eye diagram may provide information of the digital data signal and the optical system such as channel noise, inter-symbol interference (ISI), performance of a transmitter, or some combination thereof within a particular signaling interval. Opening the eye generally refers to improving the eye diagram, which may occur through adjusting an offset of the input signals.
The eye diagram may also be used to observe the cross-point. The cross-point is the point on the eye diagram where the transitions from high to low and low to high occur. For example, a digital low is often represented by a 0 volt signal and a digital high is represented by a 1 volt signal. Accordingly, in an ideal system, the cross-point would be observed at 0.5 volts.
One important task performed by a receiver of the transceiver is to determine if the bits of the received data signal represent a digital 0 (low) or a digital 1 (high). In order to perform such a task, circuitry in the post-amplifier reads the received data signal and makes the determination. In an ideal system with a cross-point at 0.5 volts, any signal of 0.49 volts and below would typically be determined to be a low and any signal of 0.51 volts or higher would typically be determined to be a high. However, it is often the case that noise and other signal offsets caused by fiber impurities, transistor mismatch, and the like cause distortion in the received signals. For example, added noise may cause a signal to be incorrectly determined as a high signal. It would therefore be advantageous to have the ability to adjust the cross-point of the received signals up or down (i.e., higher or lower than 0.5 volts) to help compensate for any signal impurities.
Another requirement might be to scan the eye diagram at different points of the XY graph to evaluate the quality of the received data quantitatively. While the scanning along the X-axis may be achieved by moving the sampling instant of the observing system, the scanning along the Y-axis requires an advertent offset to be inserted that can be achieved by crosspoint adjust circuits.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some embodiments described herein may be practiced.